Formation of silicon alloy coatings



April 1932' L. H. MAiasHALL FORMATION OF SILICON ALLOY COATINGS FiledDec. 27, 1927 silicon alloy coatinfgs Patented A pr. 12, 1932 UNITEDSTATES. PATENTS OFFICE I LESLIE H. MARSHALL, OF COLUMBUS, OHIO, ASSIGNORT0 TEE 'IEOHNIHET COIPANY,

OF COLUMBUS, OHIO, A CORPORATION OF OHIO romm'rrou or srmcon ALLOYooa'rm'es Application filed December 27, 1827. Serial No. 242,888. g

The invention relates to the production of a silicon alloy coating orlayer on the surface of articles to which it is desired to givesuperficially the characteristics of silicon or its alloys.

- It has con alloys are remarkably resistant to many types ofcorrosion.Alloys containing up to 20% of silicon are marketed for use in theproduction of corrosion resisting articles, some of these alloys beinknown under trade names such as Duriron, antiron and Ironac. Thesealloys find wide use particularly in the chemicalindustry and havedemonstrated their remarkable resistance to corrosion, especiallysulphuric acid corrosion. These alloys, however, have to be cast toshape and are extremely hard and brittle and not susceptible of beingmachined. Also, even the heaviest sections are apt to fracture becauseof their brittleness. Accordingly the fieldof application of thesealloys is limited. In view of these facts it has been proposed to formon articles which it was desired to protect rom corrosion; but, as faras I am aware, no successful method of carrying out this proposal hasbeen found prior to my present invention, the coatings previouslyproduced havin been characterized by such hardness and brittleness as torender them unduly fragile. Furthermore, the relatively hightemperatures involved in thetreatments heretofore proposed have renderedit extremely difiicult to avoid oxidation both of the silicon and of thesurface to be coated.

or alloyed.

I have discovered that the brittleness and fragile character of thesilicon alloy coatin heretofore produced is due to the relative hightemperatures of the treatments whic have been employed. Accordingly oneof the chief objects of the present invention is the provision of aprocess of forming silicon alloy coatings which involves the. use ofvery moderate temperatures, with resultant production of relativelystrong-,tough coatings.

Another object of the invention is the rovision of a process of formingsilicon a loycoatings that is es ecially applicable to thecoating ofmalleab e iron castings.

been known for some time that sili- A further object of the invention isthe provision of a process of forming silicon alloy coatings whichaffords a non-oxidizing atmosphere under conditions readily maintainedin commercial operation.

Another object of the invention is the provision of a process of formingsilicon alloy coatin s in which the surfaces of the article treated arerendered clean by ositive fluxing action, with resultant uni orm coatingthereof,

Other objects more or less incidental or ancillary to those noted abovewill appear from the following description.

Speaking generally, my improved process involves enclosing the articleto be 'ven the silicon alloy coating with material w ich will whenheated to a relatively moderate temperature provide (a) a non-oxidizinatmosphere ca able of displacing air from t e space surroun ing thearticle before the tem erature is high enough to causesubstantiaFoxidation of the article to be coated and/or (6) a fluxinactionon the article to be surin c ean surfaces thereof, and c) acompoun of silicon adapted to vaporize by the time the moderatetemperature of the treatment is attained and further adapted, while inthe vapor state, to react or dissociate with resultant deposition ofsilicon on the article treated and the formatlon of a silicon alloy alsocapable in the vapor or gaseous state of reacting with the silicon orits compound toform a silicon compound which in turn assumes the vaporstate at the moderate temperature of the treatment and in that statereacts or dissociates with the resultant deposit of silicon uponthe-article to form an alloy coating thereon.

In order that my invention and the method coated inof practicing it maybe clearly understood, I will now give concrete examples, referencebeing had to the accompanying drawing which shows more or lessdiagrammatically a simple form of apparatus suitable for the carryingout of the process. The articles to be coated, for example, may be.formed of commercially pure (Armco') iron. These articles, which aredesignated by the numeral 1 in the drawing, after being thoroughlycleaned by sand blasting or other means, are introduced into a suitablecontainer 2 and imbedded, as shown, in a packing 3 composed of a mixtureof crushed silicon (97% silicon) of a size to pass a six mesh screen andincluding fines, an equal volume of silica sand of about thirty meshsize, (Ottawa silica sand is suitable) and crushed ferric chloride,preferably but not necessarily anhydrous, to the amount of 5% by weightof the entire packing. The container 2 has a tightly closed bottom sothat vapors or gases cannot escape. Also, a layer of the packingmaterial of considerable thickness is covered over the topmost articlesto be treated to protect them from any gases that might enter throughthe top of the receptacle. The articles, however,

are further protected from gases by the provision of a closely fittingcover 4 which is sealed at the edges of a luting 5 of a refractorymaterial such as powdered flint mixed with water to forma paste. 2 maypreferably be made of low carbon, high chromium (25% chromium) iron,which is adapted to resist oxidation and deformation under the action ofheat. The container and contents are now placed in a furnace and heatedto temperatures from 1350 F.

to 1400 F. and at such temperatures held for three hours. The containeris then removed from the furnace and allowed to cool to roomtemperature. When the articles being treated have cooled they areremoved,

from the container and packing and immediately washed in hot water andthen boiled in a 10% sodium bicarbonate solution for onehalf hour toneutralize and remove any salts remaining on their surfaces. Finally thetreated articles are washed and dried. I

The articles thus treated are found to be uniformly covered with asilicon alloy coating about five one-thousandths of an inch thick, thealloy of the coating being relatively high in'silicon at its surface,with the percentage of silicon decreasing toward the interior and thecoating being relatively strong and tough.

As further illustrating the application. of my invention aswell as,indicating something of the possible range of variation of the varioussteps, I give the following additional example involving the coating ofmalleable iron castings. Following the general procedure described inconnection with the first example, the malleable castings, having firstbeen The container cleaned, as by'sand blasting, are introduced into thetreating container and imbedded in a packing of the followingcomposition:

50% of 97% silicon, crushed to pass a six mesh screen, including thefines that pass.

45% of gravel, of a size to pass a six mesh screen and including finesthat pass. 5% of anhydrous ferric chloride.

- The containers thus packed are placed in the furnace and heated totemperatures ranging from 1300 F. to 1350 F. for a period of six hours.Thereafter the containers are removed from the furnace and after theyhave bee-n allowed to cool the castings are removed from the packing andas in the first example cleansed of any halide salts adhering to theirsurfaces. The castings are found to have a satisfactory coatingvofsilicon alloy as in the first instance. Special significance and valueattaches to this last procedure because of the low temperaturesemployed. Thoroughly annealed cast iron can be heated as high as itscritical temperature, that is around 1350 F., without alteringitsstructure. As soon, however, as the temperature goes much above thispoint, say to 1400 F., the graphitized carbon begins to be reabsorbed bythe iron which loses its softness and ductility. Such castings wouldhave to be reannealed in order to be brought back to their desiredcondition. Such reannealing is avoided by the procedure last describedabove because thetemperature is kept down to or below 1350 F. In manyinstances it would be desirable to apply silicon alloy coatings tomalleable iron castings after they have been machined. The lowtemperature which characterizes my improved process makes it possible todo this without the necessity of reannealing the castings after thecoating treatment.

Without giving further specific examples in detail, it may be noted thatthe procedure first described above may, within the scope of the presentinvention, be varied considerably with respect to materials employed,the time of treatment, temperatures of treatment, and in other respects.Thus various materials may be used to supply the silicon. For example,in the procedure described ferro-silicon containing 50% silicon may besubstituted for the 97% silicon specified if the temperature oftreatment for three hours is raised to 1500 F. Furthermore, ferrosiliconwith still lower percentages of silicon can be employed if still highertemperatures are employed. Coatings may be secured using as high as 99%of the sand or other refractory constituent, though in such cases thecoating secured is not quite as thick as that secured with the preferredproportions first described. If the sand or other diluent is omitted thereacting materials, if they contact with the article being coated, tendto sinter on it and render the surface of the coatused as the 1p tinuedfor t irty hours or more.

ing somewhat porous so that the surface has a tendency to retainmoisture and promote corrosion when the coated article is put into use.This difiiculty is likely to be serious if there is less than 30% of thediluent in the packing. By the use of a suitable amount of diluentmaterial it is feasible to work up to temperatures of about 1700 F. Onthe other hand, I have found that thin alloy coatings can be produced attreating temperatures as low as 1000 F. if undiluted 97% silicon crushedto pass a six mesh screen be acking and the heatin be conowever, theprocedure first described is preferred both because it gives a densercoating more resistant. to corrosion than do thetreatments at the lowestand highest temperatures mentioned, and because the operations can becarried on with suflicientvease at the intermediate temperatures.Carbon, aluminum or other alloying elements may be present in thesilicon or ferro-silicon used, but such diluents may slow up the processof the coating. Diluents other than sand, such as alumina, mag

nesia or other refractory materials may be used. It is noted furtherthat the particle size of the crushed silicon packing material is amatter of some practical importance. Pulverized material of courseresents a larger surface and yields a coating somewhat more quickly thanthe six mesh material specified, but the sintering of the packing on thearticle coated is more pronounced when the packing material is powdered.On

the other hand larger particle size than the six mesh size can beused,but such material tends to correspondingly slow up the coating action.Generally speaking, t e silicon or. ferro-silicon crushed to pass a sixmesh screen (and including such fine material as passes the screen alongwith the coarse), gives the best results.

- The material used as a flux and silicon carrier should be volatile atthe temperature of treatment, and preferably, as has been indicated, itshould volatilize below the temperature at which either the ackingmaterial or the article to be coated oxldizes materially. I have foundhalogen compounds that vaporize or at least have an appreciable vaporpressure or evolve halogen gas, below about 700 F. to be satisfactory.Some of such compounds merely vaporize, wholly or in art, at temperaturebelow 700 F. and thus isplace any air present in the container with anon-oxidizing atmosphere, the vapors being heavier than the air.Examples of halogen aluminum chloride, antimony chloride, ar-

senic chloride, mercuric chloride, phos horous trichloride, carbontetrachloride, su phur monochloride and bismuth chloride. Am-

monium chloride also vaporizes below 700 F.

and within the same temperature range dissociates with formation ofammonia and b drochloric acid, both of which are gaseous and adapted todisplace air from the treating container. Also, the hydrochloric acidhas an etching efiect on iron and gives a clean surface, and at thehigher temperatures of the coating treatment it dissociates to ermitsome chlorine to react to form silicon ch oride. Others of the halogencompounds liberate halo ens on heating and they are thereforeparticularly suitable because the halo en not only displaces the air andis non-oxidizing but also has a fluxing or etching action on the articlebeing treated. Examples of these latter compounds, in addition to ferricchloride are lgold (auric) chloride, cupric chloride, sulp urdichloride, sulphur tetrachloride and phosphorous pentachloride. In theforegomg specific examples the ferricchloride can be replaced, forexample, by a similar amount of cupric chloride. The ammonium chlorideis not .so desirable for my purposes because the ammonia evolved tendsto react objectionably with iron being coated. Halogen compounds otherthan chlorides also serve my purpose. Thus 5% of anhydrous ferric bromide can be used in place of the ferric chloride given in "the aboveexample though the ferric chloride is preferable because of its lowercost. Furthermore, the ferric chloride need not be introduced into thetreating container as such. Thus in the packing material there maybesubstitut'ed for the 5% of ferric chloride, 10% (by weight) of amixture of equal amounts by weight of owdered ferroussulphate-(preferabl anhydrous) and powdered sodium chlori e. On heatingthe acking, these two compounds react to ive erric chloride and sodiumsulphate. known method of forming ferric chloride, b reaction within thepacking, might be use However, this method of introducing the ferricchloride usually contaminates the packing as with the sodium sulphatein'the example just given, and this is ordinarily undesirable.

The base metal on which the coating is produced may vary widely. Forexample, satisfactory coatings can be made on articles of malleable castiron, gray cast iron, ferrous alloys and steel. The coatings on lowcarbon steel, that is to say, steel with up to about 0.2% carbon, aresomewhat more tenacious than those on high carbon steels. Obviouslyarticles made of higher carbon steel can be coated with perhaps bettereffect, if the articles are first superficiall decarbonized by knownmethods. Also ot er metals such as copper, brass, bronze, nickel,molybdenum, chromium and tungsten can be coated. Thus articles of coppercan be coated as were the iron articles in the example first givenabove, usin a temperature of 1300 F.

hat has been said above sufliciently indicates the considerable range oftemperatures which can be employed under the varying ny other emperaturesuitable of treatment being dey by the concentration of the amount ofinert diluent employed 6 character of the base material coated. etreatment temperatures which I have found useful range from 1000 F. to1700 F.

In this connection it is to be observed that the packing material itselfneed not be heated to thefull treating temperatures herein specifiedsince the silicon halide (or halides) is formed and volatilized at lowertemperatures, in fact well below 700 F. in the case of the chlorides.Thus where the article to be coated can be heated by passing an electriccurrentthrough it, it will suffice if said article is thus brought tothe treating temperature though the packing material surrounding the genevolved, reacts with the silicon to article does not attain thattemperature.

The duration of the treatment is more or less variable, dependingupon-the temperature and the'composition of the packing material. Thisalso is apparent from the foregoing description.

I am not able to say with certainty precisely what goes on in thecarrying out of my process, but as at present advised I believe that thefollowing actions occur. In the first stages of the heating, that iswhile the temperature is rising to say700 F., the ferric chloride orother halogen compound employed either vaporizes or dissociates withevolution of chlorine (or other halogen). In either case, the vapor orhalogen gas formed displaces any air present in the treating containerand thus obviates or at least minimizes oxidation both of the article tobe coated and of the coating material. In case chlorine or other halogenis evolved even if some slight oxidation of the article being coated andof the silicon should occur, the fluxing action of the chlorine cleansesthe surface of the resulting oxide scale. The halogen compound in thevapor state, or the chlorine or other haloform silicon halide which alsovaporizes. These latter actions occur at moderate temperatures and, asabove noted, at temperatures well below 700 F. in the case of thesilicon chlorides. Hence the silicon halide, as well as the originalhalide, may provide non-oxidizing and fluxing conditions beforeoxidizing temperatures are reached and maintain said conditions as thetemperature'rises. Taking a concrete case, ferric chloride when heatedevolves chlorine by dissociation as follows: QFeCI QFeCl Cl The chlorinethus liberated may react with the silicon thus:

, Si+ 2Cl SiCl This reaction is reversible and in the presence of thearticle to be coated, the silicon chloride would dissociate to formsilicon and chlorine.

The silicon would immediately alloy with the metal (iron for example) ofthe article being treated at the temperature of the treatment and thehalogen compound would continue to dissociate. The chlorine freed by thedissociation of the silicon chloride would then react with more siliconat points more remote from the iron article and the deposition ofsilicon would thus continue.

Possibly some of the chlorine evolved may react with the iron of thearticle being coated thus:

Cl +Fe=FeC1 This ferrous chloride would have an appreciable vaporpressure at the temperature of the treatment. The dissociation of thisvapor in the neighborhood of the packing material would permit thelatter to alloy with the iron of the vapor and the chlorine thusliberated would react to form silicon chloride. Thus, in any case, thechlorine acts as a carrier of silicon, permitting this element to passthrough the vapor stage at a lower temperature than would otherwise bepossible.

Whatever the precise actions may be that go on during the process, theresult is the formation of a complete and highly uniform coating ofsilicon or silicon alloy on the article treated, so that the surface ofthe coated article takes on the characteristics of the silicon alloys,especially a bright appearance, great resistance to weathering, acidcorrosion (particularly that of sulphuric acid solution), soil corrosionand high temperature oxidation up to 2000 F. Furthermore, the coatingsecured is relatively tough and durable. Thus by my improved process anarticle is produced adapted to many commercial uses. This result,furthermore, is secured with a process which lends itself readily toproduction under commercial conditions. In particular, the

prevention of oxidation by the simple expedient of introducing into thepacking material a substance ada ted to form a vapor or gas capable ofdisplacing air from the treating space is easily accomplished underworking conditions readily attainable. The articles to be coatedare-packed in the treating container under ordinary conditions withoutexclusion of air, difliculties that would be incident to theintroduction. into the treating container of a non-oxidizing gas as suchare avoided, approximately atmospheric pressure is used throughout theprocess, and where the-halide of the metal to be coated is used as theflux or carrier further advantage is attained. Again, the conversion ofthe silicon into a compound vaporizable at moderate temperatures andcapable of reacting or dissociating in the presence of the article to becoated with resultant deposition of silicon both makes the processsusceptible of operation under working conditions that can easily bemaintained in commercial operation and also renders the processavailable for work that could not be accomplished at highertemperatures, such for example as the silicon coating of malleable ironcastings without the necessity of subsequent malleabilizing treatments.

The advantages attaching to the use of a packing or treating materialthat evolves a non-oxidizing gas or vapor, and particularly a halogengas, are not limited to the forming of silicon or silicon alloy coatingsbut can be realized in forming alloy coatings of other substances.'Thus, for example, I am able to employ this same feature informing-chromium alloy coatings, as set forth in my copendingapplication Serial No. 242,837, filed December 27', 1927. In the saidchromium coating process, as preferably practiced, the article to becoated which may for example be formed of low carbon steel, is enclosedwith a packing including metallic chromium, or an alloy or compoundthereof, and a substance, such for example as ferric chloride, capableof conversion to a non-oxidizin vapor or of evolving a non-oxidizing andlluxing gas at temperatures below 700 F. Heat is then applied to causesuch vaporization or evolution of gas and the packing and article arethen further heated to temperatures between 1500 F. and 2000 F until acoating of chromium alloy is formed on the article. At the lattertemperatures the halogen gas or vapor reacts with the chromium to formchromium chloride which in .turn reacts or'dissociates with theresultant deposit of chromium on the article to be coated.

In characterizing herein the gas or vapor in the treating container asnon-oxidizing, that term is, of course, used in its strict or limitedsense indicating that the gas or va or is of such a nature that it willnot form a 1m of metallic oxides on the article to be coated.

\Vhile I have set forth some of the variations of materials,temperatures and times of treatment which may be used in the carryingout of my invention, it should be understood that I have not attemptedto point out all variations of this character but that the inventioncomprehends all modifications within the scope of the appended claims.

What I claim is 1. The process of forming alloy coatings of siliconwhich includes the steps of enclosings the article to be coated with apacking containing silicon or an alloy of silicon and i the metal ofwhich the said article is constituted; applying heat to the said articleand packing; permeatingthe space around the article with sufficienthalogen or halogen compound gas or vapor. before the temperature of thearticle has been raised above 700 F. to displace the air from said spaceand thereby prevent oxidation ofthe reacting metals present; .andthereafter continuing the heating of the packing and article to holdsaid article at temperatures between 1000 F. and 17 00 F. in thepresence of halogen or halogen compound gas or vapor until a coat-= ingof silicon alloy is formed on the article.

2. The process of forming alloy coatings of silicon which includes thesteps of enclosing the article to be coated with a packing containingsilicon or an alloy of silicon and the metal of which the said articleis constituted; applying heat to the said'article and packing;permeating the space around the article with sufficient halogen orhalogen compound gas or vapor before the temperature of the article hasbeen raised above 7 00 F. to displace the air from said space andthereby prevent oxidation of the reacting metals present; and thereaftercontinui the heating ofthe packing and article to hold said article attemperatures between 1000 F. and 17 00 F. in the presence of halogen orhalogen compound gas or vapor until the latter reacts with the siliconto form a silicon halide and the latter reacts or dissociates in thepresence of the article to form a coating of silicon alloy on thearticle.

3. The process of forming alloy coatings of silicon which includes thesteps of enclosing the article to be coated with a packing containingsicilon or an alloy of silicon and the metal of which the said articleis constituted and a halogen or halogen compound adapted to evolve anon-oxidizing as or vapor at temperatures below 7 00 heating the.packing and contained article to cause such evolution of gas or vapor insufiicient amount to permeate the space surrounding the article,

substantially displace the air from said space and thereby preventoxidation of the reacting metals present; and further heatin the packingand article to hold said artic e at temperatures between 1000 F. and1700 F. in

the presence of halogen or halogen compound gas or vapor until a coatingof silicon alloy is formed on the article. I

4. The process of forming alloy coatings of silicon which includes thesteps of enclosing the article to be coated with a packing containingsilicon or an alloy of silicon and the metal of which the said articleis con-- stituted and a material adaptedto evolve a halogen gas attemperatures below 700 F.; heating the packing and contained article tocause such evolution of gas in suflicient amount to permeate the spacesurrounding the article, substantially displace the air from said spaceand thereby. prevent oxidation of the reacting metals present; andfurther heating the packing and article to hold said article attemperatures between 1000 F. and '17 00 F. in the presence of halogen orhalogen ing the article to be coated with a packing containing siliconor an alloy of silicon and the metal of which the said article isconstituted, an inert diluent and a halogen or halogen compound adaptedto evolve a nonoxidizing gas or vapor at temperatures below 700". F.;heating the packing and conj tained article to cause such evolution ofgas or vapor in suflicient amount to permeate the space around thearticle, substantially .displace the air from said space and therebyprevent on'dation of the reactingimetals present and further heating thepac ing and article to" hold said article at temperatures between 1000F. and 1700 F. in the .presence of halogenor halogen compound gas orvapor until a coating of siliconalloy is formed on the article.

6. The process of forming alloy coatings of silicon which includes thesteps of enclosingthe article to be coated with a packing containingsilicon or an alloy of silicon and the metal of which the said articleis constituted; applying heat to the said article and packing;permeating the space around the article with suficient halogen orhalogen compound gas or vapor before the temperature of the article hasbeen raised above 700 F. to displace the air from said space and therebyprevent oxidation of the reacting metals present; and thereaftercontinuing the heating of the packing and article to hold said articleat temperatures between 1000 F. and 1700 F. in the presence of halogenor halogen compound gas or vapor until a coating of silicon alloy isformed on the article; removing the article from the packing; andremoving from the surface of the article adherent halide sa ts.

7. The process of forming alloy coatings of silicon which includes thesteps of enclosing the article to be coated with a packing containingmaterial consisting of upwards of 90% silicon, an inert diluent and ahalogen or halogen compound adapted to evolvea nonoxidizing gas or vaporat temperatures be- ;low 700 F.; heating the packing and containedarticle to cause such evolution of gas or vapor in an amount suflicientto permeate the'space surrounding the article, substantially displacethe air from said space and thereby prevent oxidation of the reactingmetals (present; and further heating the packing an article to hold saidarticle at temperatures between 1350 F. and 1500 F. in 'the' presence ofhalogen or halogen compound or vapor until a coating 0 silicon alloy isformed on' the article.

8. The process of forming alloy coatings of silicon which includes thesteps of enclosing the articleto be coated with a packing containingsilicon or an alloy of silicon and the metal of which the saidarticle isconstituted and a halide adapee'd to evolve a halogen gas attemperatureslow 700 F.; heating the packing and contained article to cause suchevolution of halogen gas in sufiicient amount to permeate the spacesurrounding the article, substantially displace the air from said spaceand thereby prevent oxidation of the reacting metals present; andfurther heating the packing and article to holdsaid article attemperatures between 1000 F. and 1700 F. in the presence of halogen orhalogen compound gas or vapor until a coating of silicon alloy is formedon the article.

9. The process of forming alloy coatings of silicon on oxidizable orcorrodible metallic ar-- ticles which includes the steps of enclosingthe article to be coated with a packing containing silicon or an alloyof silicon and the metal of which the article is constituted and ahalide of the metal to be coated which is adapted to evolve a halogengas at temperatures below 700 F. heating the packing and containedarticle to cause such. evolution of halogen gas in sufiicient amount topermeate the space surrounding the. article, substantially displace theair from said space and thereby prevent oxidation of the reacting metalspresent; and further, heating the packing and article to hold saidarticle at temperatures between 1000 F. and 1700 F. in the presence ofhalogen or halogen compound gas or vapor until a coating'of siliconalloy is formed on the article.

10. The process of forming alloy coatings of silicon which i'ncl des thesteps of enclosing the article to be 'poated with a packing containingferro-silicon and a halogen or halogen compound adapted to evolve anon-oxivapor until a coating of silicon alloy is formed on the article.

11. The process of forming alloy coatings of silicon which includes thesteps of enclos ing the article to be coated with a packing containingthe silicon or an alloy of silicon and the metal of which the saidarticle is constituted and ferric chloride; heating the packing andcontained article to evolve chlorine from the ferric chloride insuflicient amount to permeate the space surrounding the article,substantially displace the air from said space and thereby preventoxidation of the reacting metals present; and further heating thepacking and article tohold. said article at temperatures between 1000 F.and 1700 F. in the presence of halogen or halogen compound gas or vaporuntil a coating of silicon v ing the article to be coated in a packingcontaining silicon or an alloy of silicon and the metal of which thesaid article is constituted and a halogen or halogen compound adapted toevolve a non-oxidizing gas or vapor at temperatures below 700 F., saidpacking being held in a container with tight side and bottom walls;-heating the packing and contained article to cause such evolution ofgas or vapor in sufficient amount to permeate the space around thearticle, substantially displace the air from said space and therebyprevent oxidation of the reacting metals present; and further heatingthe packing and article to hold said article at temperatures between1000 F. and 17 00 F. in the presence of halogen or halogen compound gasor vapor until a coating of silicon alloy is formed on the article.

13. The process of forming on oxidizable or corrodible metallic articlescoatings of material adapted to alloy 'with the base material of thearticle, which process includes the i steps of enclosing the article tobe coated with a packing containing a coating material; applying heat tothe said article and packing; permeating the space surrounding thearticle with a halogen or halogen compound gas or vapor before thetemperature of the article has been raised above its oxidation pointtodisplace the air from said space and thereby prevent oxidation of thereacting metals present; and thereafter continuing the heating of thepacking and article to temperatures above said oxidation point in thepresence of halogen or halogen compound gas or vapor until the desiredcoating is formed 'on the article.

14. The process of forming on oxidizable or corrodible metallic articlescoatings of material adapted to alloy with the base material of thearticle, which process includesthe steps of enclosing the article to becoated with 'a packing containing the coating material and a halogen orhalogen compound adapted to evolve a non-oxidizing gas or vapor attemperatures below 700 F.; heating the packing and contained article tocause such evolution of gas or vapor in sufiiclent amount to permeatethe space around the article, substantially displace the air from saidspace and thereby prevent oxidation of the reacting metals present; andthereafter continuing the heating of the-packing and article to holdsaid article at temperatures above 700 F. in the presence of halogen orhalogen compound gas or vapor until the desired coating is formed on thearticle.

15. The process of forming on oxidizable or corrodible metallic articlescoatings of material adapted to alloy'with the base material of thearticle, which process includes the steps of enclosing the article to becoated with a packing containing the coating material and a materialadapted to evolve a halogen as or vapor at temperatures below 700 i1;heating the packing and contained article to cause such evolution of gasor vapor in suf ficient amount to permeate the space around the article,substantially displace air from the said space and thereby preventoxidation of the reacting metals present; and thereafter continuing theheating of the packing and article to hold said article at temperaturesabove 700 F. in the presence of halogen or halogen compound gas or vaporuntil the desired coating is formed on the article.

16. The process of forming alloy coatings of silicon which includes thesteps of enclosing in a treating chamber the article to be coatedtogether with material including silicon or an alloy of silicon and themetal of which the said article is constituted; applying heat to thesaid article and material; permeating the space around the article withsufficient halogen or halogen compound gas or vapor before-thetemperature of the article has been raised above 7 00 F. to displace theair from said space and thereby prevent oxidation of the reacting metalspresent; and thereafter continuing the heatin of the said material andarticle to hold sald article at temperatures between 1000 F. and 17 00F. in the presence of halogen or halogen compound gas or vapor until acoating of silicon alloy isformed on the article.

meating the space around the article with suf-,

ficient halogen or halogen compound gas or vapor before the tem eratureof the article has been raised above 00F. to displace the 17. Theprocessof forming alloy coatings V of silicon which includes the stepsof enclosair from said space and thereby prevent oxidation of thereacting metals present; and thereafter continuing the heatin of thesaid material and article to hold said article at temperatures between1000 F. and 1700 F. in the presence of halogen or halogen compound gasor vapor until a coatin of silicon alloy is formed on the article; anremoving from the surface of the article adherent halide salts; v a

18. The process of forming an oxidizable or corrodible metallic articlescoatings'of material adapted to alloy with the base material of thearticle,-which process includes the ste s of enclosing in a treatingchamber the artic e to be coated together with material including acoating substance; applying heat to the said article and material;permeating the space surrounding the article with a halogen or halogencompound gas or vaporbefore the halogen or halogen compound gas or vaportemperature of the article has been raised above its oxidationtemperature to displace the air from said space and thereby preventoxidation of the reacting metals present; and thereafter continuing theheating of the said material and article to temperatures above saidoxidation point in the presence of until the desired coating is formedon the article. a y 7 In testimony whereof, I hereunto afiix mysignature. x

' LESLIE H. MARSHALL.

